Hitherto, there are known the following methods for heating steel materials in various furnaces such as heating furnaces, annealing furnaces and heat-treating furnaces, for example, under a non-oxidizing condition.
(1) Radiant Tube Heating Method ("Recent Practical Combustion Technology": edited by The Iron and Steel Institute of Japan, (1983), p. 31)
According to this method, a radiant tube is disposed in a heating furnace, etc., a high-temperature combustion exhaust gas from a burner unit or a gas having high temperatures raised by the exhaust gas is supplied to the radiant tube, and a steel material is heated with heat radiating from an outer wall of the radiant tube toward the interior of the furnace. Because a furnace atmosphere contacting the steel material can be freely set, it is easy to bring the furnace atmosphere into a non-oxidizing state.
(2) Direct-Flame Reduction Heating Method (The 88-th Nishiyama Kinen Gijutsu Koza (Nishiyama Memorial Technology Lecture), (1983), p. 75)
According to this method, a reducing flame formed in an outer portion of a flame produced by a burner unit is directly put into contact with a steel material to heat it in a reducing atmosphere.
(3) Two-Layer Atmosphere Combustion Method (Nippon Kokan Technical Report, No. 120 (1988), p. 24)
According to this method, a steel material is heated with such a two-layer atmosphere adjustment that a steel material is surrounded by a non-oxidizing atmosphere produced by imperfect combustion, and at the same time secondary combustion is performed in a not-yet-burnt gas area outside the non-oxidizing atmosphere.
Although the above heating methods are adapted for use with heating furnaces, etc. for steel materials, similar methods are also employed in heating nonferrous metals such as aluminum and copper.
The conventional heating methods stated above however have problems as follows.
(1) Radiant Tube Heating Method
This method is very superior in that an oxidizing gas containing H.sub.2 O produced by combustion, extra O.sub.2 during combustion, etc. can be completely isolated from the furnace atmosphere.
But the following problems are encountered in this method.
a) When the temperature of a heating furnace is as high as 1200.degree. C., for example, there is no effective tube endurable against such a high temperature. In other words, the radiant tube itself is broken due to thermal stresses and high-temperature creep; hence the life of the radiant tube at the high temperature is short.
b) Because the burner unit performs combustion in a narrow space of the radiant tube, there is a limit in combustion capacity of a burner itself.
(2) Direct-Flame Reduction Heating Method
This method requires the reducing atmosphere to be formed near the steel material. The following problems are therefore encountered in this method.
a) From the point of operation, there are restrictions in, e.g., the surface temperature (900.degree. C. or below) of the steel material and combustion conditions (load, air/fuel ratio, burner capacity), etc.
b) From the point of equipment, there are restrictions in, e.g., the distance from the surface of the steel material to the burner.
c) Thermal efficiency is poor because only part of combustion heat available from fuel is utilized. For the above reasons, this method cannot be applied to, e.g., heating furnaces for rolling steel materials.
(3) Two-Layer Atmosphere Combustion Method
The following problems are encountered in this method.
a) Because of forming a two-layer atmosphere, burner layout in the furnace suffers from restrictions in, e.g., that a roof burner and a side burner cannot be used in a combined manner. This raises a problem in evenness of the heating temperature when large-size steel materials are to be heated.
b) A heating ability per unit volume of the furnace is smaller than conventional burners. The furnace volume must be therefore increased, resulting in a larger size of the furnace.
c) When a combustion load fluctuates, the non-oxidizing atmosphere tends to easily break. Thus, it is difficult to apply this method to furnaces undergoing large load fluctuations because the non-oxidizing atmosphere easily changes to an oxidizing atmosphere in such a condition.
Furthermore, the method to create a non-oxidizing atmosphere near a combustion area while burning gas, like the direct-flame reduction heating method and the two-layer atmosphere combustion method, is subject to severe restrictions in the furnace temperature and combustion conditions. More specifically, to obtain a non-oxidizing atmosphere on condition that the steel material temperature is higher than 1200.degree. C., the composition of a combustion gas is required to meet the relationships of CO/CO.sub.2 &gt;3.1 and H.sub.2 /H.sub.2 O&gt;1.2. When a coke furnace gas, for example, is used as the combustion gas, the gas must be burnt under a condition of air ratio&lt;0.5. But even if the combustion is carried out while meeting those restrictions in operation, it would be difficult to stably maintain a completely non-oxidizing atmosphere near the surface of the steel material, and to prevent sufficiently oxidization of the surface of the steel material in practice.
The present invention has been developed in view of the problems stated above, and intends to quickly raise the temperature of a combustion exhaust gas for thereby improving thermal efficiency. And according to the present invention, it is possible to maintain a non-oxidizing atmosphere by using nitrogen or argon as an atmosphere gas, to create a reducing atmosphere by mixing a reducing gas to the atmosphere gas, and to cut down the cost by recovering such an atmosphere gas for reuse.